The rubber flex line is a good place to start if you cant find any other issues!...i had that problem with an 82 camaro I had.it actually locked the front driver side up!...this was any easy fix...to bad I went through new rotors,calipers,and pads before I was told about the lines!!...

Sometimes the rubber brake hoses can suffer a blowout internally on the inside liner and a flap of rubber will drop down preventing the fluid from returning properly. This could cause the problem you described.

At any rate replacing the rubber lines is always a good idea if you are not sure of their condition.
Maybe consider replacing them with braided lines which are much stronger and give a much better pedal feel.

Sometimes the rubber brake hoses can suffer a blowout internally on the inside liner and a flap of rubber will drop down preventing the fluid from returning properly. This could cause the problem you described.

At any rate replacing the rubber lines is always a good idea if you are not sure of their condition.
Maybe consider replacing them with braided lines which are much stronger and give a much better pedal feel.

I had to replace all the steel lines busted a whole the rear and after looking and pushing a screw driver through in the front with out any effort it was time to replace . As for pad all new and next will be the flex ones but guess what I take it for a ride to test and mow its bogging when you give it gas and idle is horrible just fixed all thAt fels likes it gonna stale and them it takes off gonna change the fuel fitter I am starting to really not like this car

Something could be lodged in the caliper between the piston and caliper. This will also cause the pads to wear at different rates. Maybe when you changed the pads you noticed one pad had more pad material left or one side was worn noticeably more the other. Example if the times it pulls it pulls right, maybe something is hindering the pistons in the left caliper. I've had an issue very similar to this and turned out small gravel was wedged in there. You don't have to be driving down dirt roads for this to happen.
Also braided brake lines are great, they give you good pressure and also help it be consistent. They will also give you you better feed back through the pedal to what the brakes are doing. Good luck